1. Field of the Invention
The present invention relates to an optical scanner for reducing shading in which an optical scanning operation is performed by a lens for scanning.
2. Description of the Related Art
An optical scanner is widely known in association with a laser printer, etc. The optical scanner has a scanned face set in conformity with a surface of an optical recording medium such as a photosensitive body and optically scanned by a laser beam. In a general optical arrangement of the optical scanner, the laser beam from a laser beam source is deflected by a light deflecting means such as a rotary polygon mirror. The laser beam is then formed by a lens for scanning as a light spot on the scanned face. Therefore, an incident angle of the laser beam incident to the scanning lens and a deflecting reflecting face of the light deflecting means is continuously changed during an optical scanning operation on one line. Reflectivity on the deflecting reflecting face and reflectivity and transmittance on a face of the scanning lens are changed in accordance with the incident angle. Therefore, intensity of the light spot on the scanned face is generally changed in accordance with the height of an image. The change in light intensity on one line in the optical scanning operation is called "shading". The shading is seriously caused when the light beam incident to the deflecting reflecting face is linearly polarized. In general, the light intensity tends to be decreased or increased on both end sides of the image in a main scanning direction in comparison with a central portion of the image.
An extinction ratio is equal to about 20 dB with respect to the laser beam emitted from a semiconductor laser and a semiconductor laser array used as a light source of the optical scanner. A great part of this laser beam is linearly polarized so that shading tends to be caused. Recently, the optical scanning operation has been performed at a wide angle to make the optical scanner compact. Accordingly, the area of a changing region of the above incident angle is increased so that shading tends to be increased. On the other hand, the high quality of an image recorded in the optical scanning operation is required. Accordingly, there is a problem about a reduction in quality of the recorded image caused by shading.